Thanks guys. Valid concerns.
As you guys may or may not know, the law of constant composition of seawater dictates that while salinity can change, the ratio of conservative ions (sodium, chloride, sulfate, etc.) does not change. By having a buildup of chloride and sodium cations, you're changing these ratios, even if you maintain your salinity by freshwater additions.
The problem I see with using any supplement other than ACS primary reagent grade material (the purest you can purchase) is that contamination (impurities in the reagents) changes these ion ratios (especially those of trace elements, Strontium, Barium, etc.) over time anyway, even with the addition of magnesium sulfate (which would cause a buildup of sulfate, still changing the ratios).
Large water changes or chemically pure reagents are the only way to prevent this that I see. I try to use both. I see no depletion of magnesium relative to overall salinity when I test for it, so I don't see a problem with my methods.